Terephthalic acid is manufactured by liquid-phase oxidation of p-xylene with air using a cobalt-manganese-bromide catalyst system (cobalt acetate, manganese acetate and hydrogen bromide) in acetic acid in the temperature range from 150 to 230° C. The relative ratio of cobalt, manganese and bromine are important, and the typical values are manganese: cobalt ratio 1:1 and cobalt: bromine ratio 1:2. In the reactor and crystallizer, most of the terephthalic acid crystallizes out from the mother liquor and is separated by filtration. The mother liquor comprises mainly acetic acid and organic compounds, for example, isophthalic acid, benzoic acid, para toluic acid, trimellitic acid, and terephthalic acid, and inorganic compounds such as cobalt, manganese and bromine compounds along with iron, nickel, calcium, chromium and sodium. Typically, a large portion of the recovered mother liquor is recycled to the oxidation reaction in order to recycle catalyst components to the oxidation reaction while purging a smaller portion to a solvent recovery system so as to maintain the level of impurities and by-products in the reaction within tolerable limits. This is called the purge stream, which is a mixture of oxygen-containing derivatives of benzene and toluene which are mono-, di- and tricarboxylic acids, aldehydocarboxylic acids, and methylol-substituted benzene or toluene or their carboxylic (benzoic or toluic) acids and which also contains catalyst components.
In a solvent recovery step, the purge stream is subjected to evaporation to remove a considerable portion of the acetic acid and water leaving a concentrate containing organic compounds together with catalyst components. The concentrate leaving can then be incinerated or treated. While such residues amount to from 2 to 25 weight percent of the aromatic polycarboxylic acid produced, such residue production annually is substantial in view of the millions of kilograms of the aromatic polycarboxylic acids produced annually. Such residues contain water-soluble aromatic carboxylic acids and water-soluble forms of the catalyst compounds. Landfill disposal of such residues is undesirable because rain and groundwater leach out the carboxylic acids, and soluble forms of the catalyst components can contaminate surface run-off water, water ways, and below-surface aquifers. Therefore, processes have been developed to recover the valuable chemicals from the purge stream of the reactor effluent, but these processes are not optimal.
U.S. Pat. No. 3,341,470 discloses incinerating the residue to oxide ash and dissolving the ash with sulphuric acid containing chloride. The manganese and cobalt components are recovered by treatment of the solution with sodium carbonate to precipitate cobalt and manganese as their carbonates. The recovered carbonates are then treated with acetic acid to produce acetates for recycle to the oxidation reaction.
A method for recovery of cobalt and manganese from the reactor effluent is given by Dynamit Nobel AG. (U. S. Pat. No. 4,490,297). The catalysts are isolated in the form of oxalates from the mother liquor stream and cobalt oxalate dihydrate and/or manganese oxalate dihydrate are restored by the action of hydrogen bromide and acetic anhydride to a form in which they are soluble in acetic acid and can be used again. Similarly, U.S. Pat. No. 4,910,175 discloses recovery of cobalt and manganese catalyst from oxidation process, by precipitation with oxalic acid and alkali metal hydroxide, followed by oxidation of precipitate in acetic acid to form cobalt and manganese acetate.
U.S. Pat. No. 6,001,763 discloses a process for recovering a solution of cobalt and manganese acetates and other valuables components of a reactor effluent of used catalyst discharged from terephthalic acid manufacturing process. The residue is pyrolized in a reaction zone forming molten metal in an electric arc and converting essentially all carbon in the residue to oxides of carbon, hydrogen and compounds vaporized in an effluent from the reaction zone. The alloy recovered is atomized to form a metal powder which is then reacted with acetic acid and aqueous hydrogen bromide to form the corresponding salts.